1. Field of the Invention
The present invention relates to a multilayered pinned layer structure for improved coupling field and giant magnetoresistance (GMR) for spin valve sensors and more particularly to a nickel iron (NiFe) based film between first and second cobalt (Co) based films pinned layer structure in one or both antiparallel (AP) pinned layers of an AP pinned layer structure or in a single pinned layer structure for improved coupling field and/or GMR of a spin valve sensor.
2. Description of the Related Art
A spin valve sensor is employed by a read head for sensing magnetic fields on a moving magnetic medium, such as a rotating magnetic disk or a linearly moving magnetic tape. The sensor includes a nonmagnetic electrically conductive first spacer layer sandwiched between a ferromagnetic pinned layer and a ferromagnetic free layer. An antiferromagnetic pinning layer interfaces the pinned layer for pinning the magnetic moment of the pinned layer 90.degree. to an air bearing surface (ABS) which is an exposed surface of the sensor that faces the magnetic medium. First and second leads are connected to the spin valve sensor for conducting a sense current therethrough. The magnetic moment of the free layer is free to rotate in positive and negative directions from a quiescent or bias point position in response to positive and negative magnetic field signals from a rotating magnetic disk. The quiescent position is the position of the magnetic moment of the free layer with the sense current conducted through the sensor in the absence of signal fields. The quiescent position of the magnetic moment of the free layer is typically parallel to the ABS. If the quiescent position of the magnetic moment is not parallel to the ABS in the absence of a signal field the positive and negative responses of the free layer to positive and negative signal fields will not be equal which results in read signal asymmetry which is discussed in more detail hereinbelow.
The thickness of the spacer layer is chosen so that shunting of the sense current and a magnetic coupling between the free and pinned layers are minimized. This thickness is typically less than the mean free path of electrons conducted through the sensor. With this arrangement, a portion of the conduction electrons are scattered at the interfaces of the spacer layer with the pinned and free layers. When the magnetic moments of the pinned and free layers are parallel with respect to one another scattering is minimal and when their magnetic moments are antiparallel scattering is maximized. Changes in scattering in response to field signals from a rotating disk changes the resistance of the spin valve sensor as a function of cos .theta., where .theta. is the angle between the magnetic moments of the pinned and free layers. The sensitivity of the sensor is quantified as magnetoresistive coefficient dr/R where dr is the change in resistance of the sensor between parallel and antiparallel orientations of the pinned and free layers and R is the resistance of the sensor when the moments are parallel.
The transfer curve (readback signal of the spin valve head versus applied signal from the magnetic disk) of a spin valve sensor is a substantially linear portion of the aforementioned function of cos .theta.. The greater this angle, the greater the resistance of the spin valve to the sense current and the greater the readback signal (voltage sensed by processing circuitry). With positive and negative signal fields from a rotating magnetic disk (assumed to be equal in magnitude), it is important that positive and negative changes of the resistance of the spin valve sensor be equal in order that the positive and negative magnitudes of the readback signals are equal. When this occurs a bias point on the transfer curve is considered to be zero and is located midway between the maximum positive and negative readback signals. When the direction of the magnetic moment of the free layer is parallel to the ABS, and the direction of the magnetic moment of the pinned layer is perpendicular to the ABS in a quiescent state (absence of signal fields) the bias point is located at zero and the positive and negative readback signals will be equal when sensing positive and negative signal fields from the magnetic disk. The readback signals are then referred to in the art as having symmetry about the zero bias point. When the readback signals are not equal the readback signals are asymmetric.
The location of the bias point on the transfer curve is influenced by three major forces on the free layer, namely a demagnetization field (H.sub.demag) from the pinned layer, a ferromagnetic coupling field (H.sub.F) between the pinned layer and the free layer, and sense current fields (H.sub.I) from all conductive layers of the spin valve except the free layer. When the sense current is conducted through the spin valve sensor, the pinning layer (if conductive), the pinned layer and the first spacer layer, which are all on one side of the free layer, impose sense current fields on the free layer that rotate the magnetic moment of the free layer in a first direction. The ferromagnetic coupling field from the pinned layer further rotates the magnetic moment of the free layer in the first direction. The demagnetization field from the pinned layer on the free layer rotates the magnetic moment of the free layer in an opposite second direction. Accordingly, the demagnetization field is counterbalanced by the sense current and ferromagnetic coupling fields.
Over the years a significant amount of research has been conducted to improve the GMR or magnetoresistive coefficient dr/R of spin valve sensors. These efforts have increased the storage capacity of computers from kilobytes to megabytes to gigabytes. It is known that when the thickness of the spacer layer is decreased the magnetoresistive coefficient of the sensor is increased so as to increase storage capacity. Unfortunately, when the thickness of the spacer layer is decreased the aforementioned ferromagnetic coupling H.sub.F between the pinned and free layer is increased. This affects the aforementioned bias point and requires that the sense current and/or the thickness of the pinned layer be changed to adjust the sense current fields and the demagnetization fields acting on the free layer. There is a strong felt need to increase the magnetoresistive coefficient of the spin valve sensor without increasing the ferromagnetic coupling field.